I don't like targeting people. I don't like having to think or know who to heal. I just let the macros do it for me. I focus my main and then use:
/cast [target=focus-target] DPSSpell for DPS
/cast [target=focus-target-target] HealSpell for Healing
The healing is done to whoever the focus' mob is beating on. No worrying about who is main. No worry about different setups. No worrying about who has aggro.

For my priest in seperate classes groups, I use:
/cast [target=TankName, nocombat] Power Word: Shield
/castsequence [target=focus-target-target, combat] reset=14 Renew, Flash Heal, Flash Heal, Power Word: Shield
It heals small, then larger, then a holy crap stop hitting my guy spell. Just mash it as much as you need it (tiny mobs, just once, larger, twice, etc).